One of the areas in neurobiology that has received considerable attention is that concerned with the neurotrophic influences of nerves on their endorgan. Work on cross reinnervation of fast and slow mammalian muscle, suggested that nerves liberate a specific substance, the neurotrophic factor, which was necessary and sufficient for determining and maintaining endorgan integrity. Recent work indicates that other factors, such as nerve impulse activity and muscular activity may be important. It is proposed that the problem can be profitably studied in a simple developing system, that of the lobster claw closer muscle. The claws of the American lobster, Homarus americanus, are heterochelous. The closer muscle of the larger crusher claw has been shown to have all slow muscle while the closer muscle of the smaller cutter claw has some slow fibers, but contains primarily fast muscle. Both claws are innervated by two excitatory axons and a single inhibitor. Thus, these homologous muscles which are innervated by an homologous pair of motor axons, have very different properties. In addition, claw placement is not genetically determined; if one claw is removed before the fifth molt, the remaining claw is always a crusher. Using standard electrophysiological techniques of intracellular and extracellular recording, the properties of the developing system will be investigated. Some of the specific lines of inquiry will deal with: 1) do the differences in muscle fiber properties between claws manifest themselves, before, during or after the fifth molt, when reversal is no longer possible 2) what are the differences in impulse activity among the axons to the claws and when do these differences become apparent. 3) Can the properties of the adult neuromuscular system be modified by such procedures as immobilization, tentotomy, and cutting of the motor nerves, and 4) can claw placement be modified in the early stages by immobilization, tentotomy or cutting the motor nerves of one claw.